cy.readFile() assertions in Cypress - Build an Automation Script

Verify contents of a JSON file using cy.readFile()

Preconditions (2)

Step 1: Use cy.readFile() to read 'cypress/fixtures/userData.json'

Step 2: Assert that the 'name' field equals 'Alice'

Step 3: Assert that the 'email' field contains '@example.com'

Step 4: Assert that the 'age' field is a number greater than 18

✅ Expected Result: The test passes if all assertions on the JSON file content are true

Automation Requirements - Cypress

Assertions Needed:

Assert exact match for 'name' field

Assert substring presence in 'email' field

Assert numeric and range condition for 'age' field

Best Practices:

Use cy.readFile() with correct relative path

Chain assertions properly with .then() or directly

Use clear and descriptive assertion messages

Avoid hardcoding file content in test, rely on fixture file

Automated Solution

Cypress

describe('Verify JSON file contents with cy.readFile()', () => {
  it('should read userData.json and assert fields', () => {
    cy.readFile('cypress/fixtures/userData.json').then((data) => {
      expect(data).to.have.property('name', 'Alice')
      expect(data.email).to.include('@example.com')
      expect(data.age).to.be.a('number').and.to.be.greaterThan(18)
    })
  })
})

This test suite reads the JSON file using cy.readFile() with the correct relative path to the fixture file.

Inside the then callback, it asserts the name property exactly matches 'Alice'.

It checks the email field contains the substring '@example.com' to verify a valid email domain.

Finally, it asserts the age is a number and greater than 18, ensuring the user is an adult.

This approach uses Cypress best practices by chaining assertions inside the promise callback and avoiding hardcoded data in the test code.

Common Mistakes - 4 Pitfalls

{'mistake': 'Using incorrect file path in cy.readFile()', 'why_bad': 'The test will fail because the file cannot be found, causing a file not found error.', 'correct_approach': "Use the correct relative path from the project root, e.g., 'cypress/fixtures/userData.json'."}

Trying to assert on cy.readFile() directly without .then()

Hardcoding expected file content inside the test instead of reading from fixture

Using loose assertions like expect(data.name).to.exist without checking value

Bonus Challenge

Now add data-driven testing to verify three different JSON files with different user data

Show Hint

Practice

1. What does cy.readFile() do in Cypress testing?

easy

A. Writes data to a file during the test

B. Reads the content of a file so you can check its data

C. Deletes a file from the system

D. Uploads a file to a server

Solution

1. Step 1: Understand the purpose of cy.readFile()

   This command is used to read the contents of a file during a Cypress test.

2. Step 2: Identify what cy.readFile() does not do

   It does not write, delete, or upload files; it only reads them.

3. Final Answer:

   Reads the content of a file so you can check its data -> Option B

4. Quick Check:

   cy.readFile() = Read file content [OK]

Hint: Remember: readFile reads, not writes or deletes [OK]

Common Mistakes:

• Confusing readFile with writeFile
• Thinking it uploads files
• Assuming it deletes files

2. Which of the following is the correct syntax to assert that a file contains the text "Hello World" using cy.readFile()?

easy

A. cy.readFile('file.txt').should('containText', 'Hello World')

B. cy.readFile('file.txt').should('equals', 'Hello World')

C. cy.readFile('file.txt').should('includes', 'Hello World')

D. cy.readFile('file.txt').should('contain', 'Hello World')

Solution

1. Step 1: Identify the correct assertion method for substring check

   The correct assertion to check if a string contains text is contain.

2. Step 2: Match syntax with Cypress documentation

   should('contain', 'text') is the valid syntax; others like 'equals', 'includes', or 'containText' are incorrect here.

3. Final Answer:

   cy.readFile('file.txt').should('contain', 'Hello World') -> Option D

4. Quick Check:

   Use 'contain' for substring assertions [OK]

Hint: Use 'contain' to check if file text includes a string [OK]

Common Mistakes:

• Using 'equals' instead of 'contain' for substring
• Using 'includes' which is not a Cypress assertion
• Using 'containText' which is for DOM elements

3. Given the file data.json contains {"name": "Alice", "age": 30}, what will this test assertion do?

cy.readFile('data.json').should('deep.equal', { name: 'Alice', age: 30 })

medium

A. Pass because the file content matches the object exactly

B. Fail because the object keys are strings in the file

C. Fail because 'deep.equal' cannot compare objects

D. Pass but only if the file is a string, not JSON

Solution

1. Step 1: Understand deep.equal assertion

   This assertion checks if two objects have the same structure and values deeply.

2. Step 2: Check if cy.readFile() parses JSON automatically

   Yes, it parses JSON files into objects, so the assertion compares objects correctly.

3. Final Answer:

   Pass because the file content matches the object exactly -> Option A

4. Quick Check:

   deep.equal compares objects deeply [OK]

Hint: Use deep.equal to compare JSON objects, not strings [OK]

Common Mistakes:

• Thinking file content is a string, not parsed JSON
• Confusing deep.equal with shallow equal
• Assuming keys must be non-string

4. What is wrong with this Cypress test code?

cy.readFile('log.txt').should('include', 'Error')

medium

A. The file must be JSON to use should assertions

B. The file path must be absolute, not relative

C. The assertion method should be 'contain', not 'include'

D. The method 'include' is correct and will pass if 'Error' is found

Solution

1. Step 1: Check the assertion method name

   Cypress uses 'contain' for substring assertions; 'include' is not a valid assertion method.

2. Step 2: Confirm correct assertion for substring

   The correct method is 'contain' to check if text includes a substring.

3. Final Answer:

   The assertion method should be 'contain', not 'include' -> Option C

4. Quick Check:

   Use 'contain' for substring assertions, not 'include' [OK]

Hint: Use 'contain' for substring checks, not 'include' [OK]

Common Mistakes:

• Using 'include' instead of 'contain'
• Thinking relative paths are invalid
• Assuming only JSON files can be read

5. You want to verify a downloaded CSV file contains the header row exactly as "Name,Age,Email". Which Cypress code correctly reads the file and asserts this?

hard

A. cy.readFile('downloads/data.csv').should('contain', 'Name,Age,Email')

B. cy.readFile('downloads/data.csv').should('equal', 'Name,Age,Email')

C. cy.readFile('downloads/data.csv').should('deep.equal', 'Name,Age,Email')

D. cy.readFile('downloads/data.csv').should('include', 'Name,Age,Email')

Solution

1. Step 1: Understand the file content and assertion needed

   The CSV file is text, and we want to check if it contains the exact header row as a substring.

2. Step 2: Choose the correct assertion for substring in text files

   contain is the correct assertion to check if the file text includes the header row.

3. Final Answer:

   cy.readFile('downloads/data.csv').should('contain', 'Name,Age,Email') -> Option A

4. Quick Check:

   Use 'contain' to check text includes header row [OK]

Hint: Use 'contain' to check CSV header text inside file [OK]

Common Mistakes:

• Using 'equal' which requires exact full file match
• Using 'deep.equal' which is for objects, not strings
• Using 'include' which is not a valid assertion